I've been asked many times if there is a threshold residential density above which many
more residents take transit. I have said it seems to be between 5 to 10
households/residential acre. Or, is there a density above which people are willing to walk
or bike? And the answer seems to be similar. The increase in these trips reduces driving
trips (and lengths, but that's another story).

San Francisco Bay Area

In 1990 the Metropolitan Transportation Commission surveyed over 10,000 households on
one or more days for each trip: reason, origin, destination and mode. I aggregated these
reports by the residential density of the householdıs zone. Residential density is the
number of households in the zone divided by the number of residential acres and is an
excellent measure of the neighborhoods we live in. It is preferred over population (or
households) per total acre because many zones contain other land uses which dilute the
measure. A zone, for instance, might contain a compact, walkable neighborhood surrounded
by much rural land, giving a population/total acre which is far below the neighborhood
density residents experience daily. While another zone containing only housing could
appear to have a higher density but really be much more sprawled out and less inviting to
pedestrians, bicycles and transit use. Or a dense urban zone which includes a handy
commercial and office area and local parks would also have unrepresentatively low
population or households per total area.

MTC's survey seems to tell us that transit use rises from 0.2-0.3 daily transit
trips/hh at 6 or 7 hh/res ac to 1.3 daily transit trips/hh at 30-50 hh/res ac, above which
it declines very slowly.

Walking also starts rising from about 0.6 walking trips/household at about 6-7 hh/res
acre, doubling to 1.4 trips/hh at 30 hh/res acre, and continues rising slowly to 1.5 trips
at 120 hh/res acre (3-5 story condos and apartments with occasional high-rises). These
increases in walking and transit use help explain why, in the San Francisco area, vehicle
miles per household fall over 40% every time neighborhood density doubles.

These results parallel the studies of driving, transit use and walking trips among
European cities. Denser cities have less driving per capita. Transit trips per capita
increase with density until they reache a maximum at moderate densities and fall or remain
constant at higher densities. Walking continues rising as density increases.

And this makes sense. When density increases driving falls as trip lengths are
shortened and more can be taken by transit or walked or bicycled. (Unfortunately, there
were too few bicycle trips in the MTC survey to lead to firm conclusions, although they
averaged about 0.1 trips/hh, rising to about 50% higher between 10-20 hh/res ac.) Transit
trips increase with density until trip distances become short enough that more are taken
by bicycle and foot. Walking trips continue to rise with density as they become more and
more convenient. The table below shows the mean values from MTC's 1990 household travel
survey.

Daily Trips by Density, from MTC's 1990 Household Travel Survey

Households/Residential Acre

< 2

2 - 2

5 - 10

10 - 20

20 - 50

> 50

Mean Hh/Res Ac

1.4

3.6

6.7

13.5

30.6

121.9

Daily Veh Trips/Hh

6.4

5.9

5.0

3.8

2.9

1.2

Daily Transit Trips/Hh

0.2

0.2

0.3

0.8

1.3

1.3

Daily Walking Trips/Hh

0.6

0.5

0.6

0.9

1.4

1.5

United States

Dunphy and Fisher ("Transportation, Congestion, and Density: New Insights,"
Transportation Research Record, No. 1552, Washington DC: Transportation Research Board,
Nov. 1996, Table 5) aggregated the 1990 National Personal Transportation Survey (NPTS) by
the population per square mile of the householdıs ZIP code. As described above, the
utility of this measure is clouded by the inclusion of nonresidential land (commercial,
industrial, parks and other open spaces) in some zones, while others are wholly
residential. Nevertheless, that is the only density measure the U.S. Census Bureau
provides by ZIP code.

The data show that auto trips decline with increasing population density, as expected.
These data indicate a near linear increase in public transit and walking-biking trips per
capita with increases in population density above 4500 persons/square mile. Transit trips
increase with density from a low of about 0.07 trips/capita below 4500 persons/square mile
(about 4 households/residential acre in a residential-only zone) steadily up to 1.03 at
the highest density of 60,000 (perhaps 100 hh/res ac in a mixed-use zone). Walking and
bicycling trips also increase steadily with density above 4500 persons/square mile from
0.3 per person to 1.55 at the highest density aggregate.